This invention relates to a low voltage electron beam excitation phosphor display apparatus and to a method of manufacturing the same.
A low voltage electron beam excitation phosphor display apparatus of the type described, comprises a single electron gun or a plurality of electron guns of a hot-cathode or a cold-cathode type arranged within the apparatus kept at a vacuum. Each electron gun (or a cathode) is for emitting a low voltage electron beam with an accelerating voltage of about 500V or less to irradiate each phosphor film (formed on each anode by mixing and heating a phosphor and a conductive material) so that the phosphor film performs high-luminance light emission to provide monochromatic or multi-color (several colors to full color) display of characters, designs, and moving pictures with an excellent balance in brightness.
In recent years, practical use has been made of various display apparatuses for displaying a condition of a device or information by means of a video image of characters, designs, and moving pictures. For example, there are widely known a cathode ray tube (CRT) extensively used as a television including a high definition television (HDTV) or as a display terminal of a personal computer, a liquid crystal display (LCD) used in a notebook-type personal computer or the like, a fluorescent indicator panel (FIP) used in a device such as a domestic (or home) video tape recorder and an on-vehicle meter to display the condition of the device or an automobile, and a plasma display panel (PDP) recently developed into a color display as a wall-hanging television. In recent years, a field emission display (FED) is proposed as a novel display apparatus and is being actively developed for practical use.
The above-mentioned display apparatuses are advantageous in that the condition and the information varying from time to time can be visually displayed and transmitted immediately under electrical control, and find a more and more increasing demand as transmitting means in the information society. The CRT has wide spread from an early time as an excellent display apparatus for a television and a personal computer and is still extensively used at present.
In the meanwhile, the CRT requires a depth corresponding to the size of a display screen. As the display screen becomes larger, the depth increases. This results in an increase in installation area and weight. In addition, the larger screen decreases the brightness of the display screen. In order to avoid the above-mentioned problems, there arises a demand for various display apparatuses in place of the CRT which requires a higher voltage and a greater electric power and lacks portability. Such demand is particularly noticeable in recent years. As such display apparatuses, so-called flat panel displays, such as the LCD, the FIP, the PDP, and the FED, capable of providing display with a small depth have been proposed and put into practical use (or involved in the process of development), as well known in the art.
Among the above-mentioned known display apparatuses, description of the prior art will hereafter be made restrictedly about those display apparatuses (CRT, FIP, FED) in which the phosphor is irradiated by the electron beam to be excited for light emission.
The CRT has a structure comprising an electron gun, a phosphor film containing a phosphor, a shadow mask arranged in an inside space kept at a vacuum. The electron gun produces an electron beam (high voltage electron beam) accelerated with a high voltage not lower than 1 kV to irradiate the phosphor so that the phosphor is made to emit light. In a color display CRT, the phosphor film has a structure such that three kinds of phosphors having emission colors corresponding to three primary colors (R, G, B) are applied in a fine stripe or dot pattern onto a panel portion to serve as a display screen. By means of the shadow mask and a deflection yoke arranged inside the apparatus, the high voltage electron beam is electrically deflected in irradiation direction to irradiate a desired part of the phosphor film so that light emission in R (red), G (green), and B (blue) is effected. The brightness (luminance) of the light emission is changed in R, G, and B by controlling the beam intensity of the electron beam to provide color mixtures as full-color display. As well known, the CRT is practically used in a television, an HDTV, and the like.
Currently, the CRT is extensively used because the ratio of display quality to the cost (namely, cost effect) is high. For example, high-brightness (for example, on the order of 300 cd (candela)/m.sup.2 or more) display is possible. Color reproducibility is excellent. Even after the use for a long time (for example, twenty or thirty thousand hours or more), brightness balance is not disturbed and color offset is suppressed. As also well known, in the past technical development leading to an excellent display quality which the CRT has at present, development of a high-brightness emission phosphor and development and improvement of a manufacturing process have taken a great role. This is described in detail, for example, in Reference (1) ("Phosphor Handbook", edited by Keikotai Dogakkai and published by Ohm Corporation, 1987). The above-mentioned reference describes the following. For a certain period of time in the past, (Zn,Cd)S has been used as a matrix composition for a green or a red phosphor. As a result of a number of trials in development of phosphors, an improved phosphor containing no cadmium but having at least an equivalent emission characteristic has been developed and used up to now. For the CRT, use is currently made of, for example, ZnS:Ag (representing the phosphor having a matrix composition of ZnS with Ag added as an activating agent for obtaining a desired emission color: similar representation will be used in the following) as a blue-emitting phosphor, ZnS:Au,Cu,Al or ZnS:Cu,Al as a green-emitting phosphor, and Y.sub.2 O.sub.2 S:Eu as a red-emitting phosphor.
Although the CRT has widely been put into practical use, it has a heavy weight and therefore lacks portability as described above. Furthermore, use of the fast electron beam requires a high voltage which in turn requires electrical parts having a high breakdown characteristic. This makes it difficult to reduce the size and weight. Thus, various problems are revealed. In order to solve the above-mentioned problems, the FIP is put into practical use. On the other hand, the FED is being developed. In the following, the prior art will be described.
Each of the FIP and the FED is an electron beam excitation phosphor display apparatus which is a so-called flat panel display having a small depth and which does not require a high voltage. Because the irradiation direction of the electron beam is not deflected, a beam deflection space along the depth is no longer required so that a thin structure can be achieved. By development and improvement of a phosphor and a phosphor film capable of performing high-brightness emission even with an electron beam produced at a low accelerating voltage and having small energy as compared with the electron beam in the CRT, it is possible to obtain a small-sized and light-weight display apparatus because an electric design for a high breakdown voltage is unnecessary. This is well known and described, for example, in Reference (1) mentioned above, Reference (2) ("Electroceramics", Vol. 24, No. 122, 1993: published by Gakukensha), Reference (3) (Takao Kishino "Vacuum Fluorescent Display", Sangyo Tosho, published in 1990), Reference (4) (Nikkei Electronics, Jan. 29, 1996, pp. 85-102), and Reference (5) (M. Urayama et al "International Electron Devices Meeting, Technical Digest, p. 16-7-1, 1995).
A light-emission display mechanism of the FIP or the FED will be briefly described. Within the display apparatus kept at a vacuum so as to allow the flight of the electron beam, a cathode (electron gun) and an anode are arranged. An electric voltage (accelerating voltage) is applied between the electron gun and the anode so that the electron beam emitted from the electron gun (of a hot-cathode type or a cold-cathode type) is made to hit or impinge upon the anode. The phosphor film is adhered to the surface of the anode. The electron beam flying towards the anode impinges upon the phosphor film so that the phosphor film is excited to emit light. The flight of the electron beam is locally controlled so that a desired part of the phosphor film alone is controllably made to emit light. Thus, characters and designs are displayed by means of light emission. The phosphor film and the anode are adhered to each other so as to be electrically conductive. As well known, adhesion is carried out by heat treatment at 400.degree.-500.degree. C. in a manufacturing process of the display apparatus.
A low voltage electron beam excitation phosphor display apparatus having an external appearance as the flat panel display is extensively put into practical use as the FIP. Specifically, the FIP provides light-emission display in response to the low voltage electron beam accelerated at a low voltage on the order of several tens of volts (in other words, having small energy) and is widely put into practical use as a character and design display apparatus, for example, as a display apparatus incorporated into electric products for domestic (or home) use such as a video tape recorder or as a display apparatus in a speedometer of an automobile. In addition, research and development are energetically carried out at present for the purpose of practization of the FED as a display apparatus for displaying a fine image comparable to that of the television. In fact, however, the prior art relating to the low voltage electron beam excitation phosphor display apparatus has the following problems (1) and (2) inherent to low voltage electron beam excitation as described also in the above-mentioned references.
(1) An irradiation electron beam has small energy because it is the low voltage electron beam. The penetration depth of the electron beam into the phosphor is restricted to the extreme vicinity of the surface of the phosphor. Although depending upon the accelerating voltage, the penetration depth is at most about 10 nm. Light emission can be caused from a restricted region corresponding to the penetration depth. Therefore, the total quantity of the activating agent contained in the phosphor can not be completely excited for light emission. Thus, high-brightness light emission is inherently difficult to obtain.
(2) The lower the accelerating voltage of the irradiation electron beam is, the easier the phosphor is electrically charged (negative). Even if the irradiation electron beam is increased in quantity for the purpose of providing the high-brightness light emission, the irradiation electron beam is subjected to repulsive force by the phosphor electrically charged negative and can not reach the phosphor. As a result, high brightness can not be achieved although the electron beam is increased in quantity.
Because of occurrence of the above-mentioned phenomenon inherent to the low voltage electron beam, it is difficult to obtain the high-brightness light emission by the use of the low voltage electron beam. In view of the above, various developments and improvements described in the following are energetically carried out at present in connection with not only the phosphor and the method of producing the same but also the method of manufacturing the display apparatus as the FIP and the FED.
Consideration will be made about the phosphor. Those phosphors put into practical use for the low voltage electron beam are restricted. For example, the above-mentioned References (2) and (3) describe that the phosphors ZnO:Zn, ZnS:Cl, ZnS:Au,Al, (Zn,Cd)S:Ag,Cl are put into practical use for green emission, blue emission, greenish-yellow emission, red-based emission, respectively.
Specifically speaking about the red-based phosphor, the red emission has much demand because it is highly visible and excellent as a display color. The phosphor (Zn,Cd)S:Ag,Cl is extensively put into practical use because the high-brightness light emission is possible. As described in Japanese Unexamined Patent Publication (JP-A) No. 104684/1989 (phosphor composition), improvement such as stabilization of the brightness is being made. However, the above-mentioned phosphor comprises cadmium which is specified as a harmful substance. As well known, there is a demand for a red phosphor containing no cadmium but performing high-brightness light emission of a comparable level. If the above-mentioned phosphor Y.sub.2 O.sub.2 S:Eu currently widely used in the CRT with the fast electron beam is used with the low voltage electron beam as the red phosphor containing no cadmium, the brightness sufficient in practical use can not be obtained. Under the circumstances, the phosphor containing cadmium has been practically used. This is well known as described in the above-mentioned references. However, development of the phosphor containing no cadmium is still continued. Disclosure is made of a composition (Y.sub.2 O.sub.2 S:Eu.aY.sub.2 O.sub.3 :1.times.10.sup.-4 .ltoreq.a.ltoreq.2.times.10.sup.-3) which comprises Y.sub.2 O.sub.2 S:Eu and oxide (Y.sub.2 O.sub.3) in a specific concentration range and which performs high-brightness light emission (Japanese Unexamined Patent Publication (JP-A) No. 310073/1995: red-emitting composition), or a composition which is obtained by partially substituting an element La for an element Y as one of main components of Y.sub.2 O.sub.2 S:Eu and which performs high-brightness light emission (Japanese Unexamined Patent Publication (JP-A) No. 310074/1995: red-emitting composition).
In addition to the development and the improvement of the phosphor adapted for the low voltage electron beam, a conductive material is added and mixed in order to suppress the electric charge of the phosphor which is the problem inherent to the low voltage electron beam. For example, admixture of In.sub.2 O.sub.3, ZnO, SnO.sub.2, or a combination (In.sub.(2-x-y) Sn.sub.x M.sub.y O.sub.x (where M is at least one member of Sb, Cd, Zn, W, Ti, and Mo) thereof is disclosed (see Japanese patent Publication (JP-B) No. 55172/1982 "Method of Producing a Fluorescent Layer for a Vacuum Fluorescent Display", Japanese Patent Publication (JP-B) No. 23912/1977 "Blue-Emitting Low Voltage Electron Beam Excitation Vacuum Fluorescent Display", Japanese Unexamined Patent Publication (JP-A) No. 168683/1994 "Vacuum Fluorescent Display", and Japanese Unexamined Patent Publication (JP-A) No. 33185/1991 "Fluorescent Material and Method of Producing the Same").
On the other hand, in manufacture of the low voltage electron beam excitation phosphor display apparatus, various approaches for improving the brightness are introduced in respective processes. For example, in a screen printing process of printing a phosphor paste (the mixture of the phosphor, the conductive material, and a binder) onto the anode site, a different material such as ZnO particles, is added as a buffer in order to avoid deterioration in crystallinity of the surface of the phosphor as a result of mechanical friction with the screen (see Japanese Unexamined Patent Publication (JP-A) No. 168683/1994 "Fluorescent Display Tube"). Alternatively, in order to effectively adhere fine particles of, for example, the conductive material onto the surface of the phosphor, a water-soluble polymer or a solvent-soluble polymer is selectively used as a binder for preparing the phosphor polymer in dependence upon a solvent used in a different process (see Japanese Patent Publication (JP-B) No. 33266/1987 "Slow Electron Beam Excitation Fluophor and Method of Producing the Same".
Furthermore, disclosure is made of another method wherein, in a heating process (for example, at about 400.degree.-500.degree. C.) carried out to decompose and remove the binder of the phosphor paste printed on the anode site so that the phosphor film is adhered, prebaking is at first carried out in air at a lower temperature in order to avoid transformation and deterioration of the phosphor (sulfide such as ZnS, (Zn,Cd)S or oxisulfide such as Y.sub.2 O.sub.2 S) and then baking is carried out in a nonoxidizing atmosphere (see Japanese Patent Publication (JP-B) No. 58618/1989 "Method of Manufacturing a Display Apparatus"). It is also disclosed that, in the low voltage electron beam excitation light emission using an oxide phosphor ZnO:Zn widely used as a high-brightness green-emitting phosphor, deterioration and fluctuation in brightness can be avoided by the use of an admixture such as tungsten oxide because the admixture such as tungsten oxide is supposed to absorb a brightness deteriorating substance (supposed to be a gas component such as H.sub.2 O, CO.sub.2) (see Japanese Patent Publication (JP-B) No. 9580/1991 "Vacuum Fluorescent Display and Method of Manufacturing the Same"). Also well known is a technical disclosure (Japanese Unexamined Patent Publication (JP-A) No. 106848/1992: Phosphors for Use in a Vacuum Fluorescent Display) teaching that, in order to more effectively obtain the similar effect, an admixture-free ZnO:Zn phosphor and an admixture-containing ZnO:Zn phosphor are admixed rather than simply admixing the admixture such as tungsten oxide. Likewise, as another approach for positively removing the brightness deteriorating factor to improve the brightness, various trials are disclosed. For example, treatment by active oxygen is followed by heat treatment in order to gasify and remove pollutants present on the surface of the phosphor as the brightness deteriorating factor (see Japanese Unexamined Patent Publication (JP-A) No. 35086/1991 "Treatment of Fluorescent Layer").
In multi-color display, use is made of a plurality of kinds of phosphors having different emission colors. If brightness balance among those emission colors is disturbed, color offset is observed. In this event, display quality is noticeably degraded. As known in the art, various modifications and improvements in manufacture have been proposed in order to keep the brightness balance within a predetermined range (for example, Japanese Unexamined Patent Publication (JP-A) No. 152782/1980: Fluorescent Material and Low Voltage Electron Beam Excitation Vacuum Fluorescent Display). In addition, various modifications and improvements have been proposed to prolong a lifetime inherent to the phosphor (taking into account the fact that the phosphor is transformed and degraded in brightness as a result of light-emission display for a long time, the term "lifetime" is defined as a time period after which the brightness is reduced to a half of an initial emission brightness) to at least several thousand hours. For example, a thin coating film or protection coating is formed on the surfaces of phosphor particles.
In the foregoing, description has been made in detail as regards the prior art relating to the low voltage electron beam excitation phosphor display apparatus in conjunction with the FIP by way of example. The FED being currently developed has similar problems resulting from the low voltage electron beam. For example, the above-mentioned Reference (4) describes current FED-related techniques including an approach to achieve high-brightness display at a low accelerating voltage (50V, 350V) by development and improvement of the phosphor, and another approach to achieve high-brightness display at a relatively high accelerating voltage (4-6 kV) using the phosphor for the high voltage electron beam for the CRT in order to avoid the problems arising from the low voltage electron beam. Unlike the FIP for displaying characters and designs (display colors typically ranging from several colors to ten colors at most), the FED is intended to display a moving picture and a video image in full color in the manner similar to the television. Therefore, the phosphor causing less color offset, in other words, excellent in brightness balance is particularly desired.
Summarizing, in the prior art relating to the low voltage electron beam excitation phosphor display apparatus, those phosphors performing light emission with a relatively high brightness are restricted and small in number. Even if the display apparatus is manufactured by simply using a phosphor having a desired composition, it is impossible to obtain the display apparatus which performs light emission display with a brightness of a practical level and which minimizes nonuniformity in brightness. In the state of the art, technical development and improvement of various kinds of manufacturing methods and conditions are energetically carried out in correspondence to the types of the phosphors (for example, sulfide, oxisulfide, oxide), as described in the foregoing.
As well known, the prior art relating to the low voltage electron beam excitation phosphor display apparatus as the FIP or the FED does not reach a technical level of the phosphor display apparatus using the high voltage electron beam, such as the one used in the television. As described above, in terms of the fineness or purity of the picture quality, the FIP has a level sufficient to display the characters and the designs. On the other hand, the FED intended to display the video image as fine as that provided in the television or the HDTV is still in the process of energetic development throughout the world and is not yet widely put into practical use. As regards multi-color display, the FIP can display several colors, at most ten colors, as described in the foregoing. On the other hand, in the FED being developed for full-color display comparable to the television, the development of the phosphor is a major technical problem as described in Reference (4) mentioned above.
The conventional red-based phosphor comprises a matrix composition of (Zn,Cd)S known as a highest-brightness (in red-based emission) phosphor and an activating agent added thereto in correspondence to a desired emission color and is extensively put into practical use. However, this phosphor disadvantageously includes cadmium which is a specific chemical substance. As described above, proposals have recently been made for the purpose of replacement of the phosphor (Zn,Cd)S (the above-mentioned Japanese Unexamined Patent Publications (JP-A) Nos. 310073/1995 and 310074/1995). In either publication, the red phosphor Y.sub.2 O.sub.2 S:Eu, which is well known but has been regarded impractical in view of the brightness, is used as a main component. The former publication (Japanese Unexamined Patent Publication (JP-A) No. 310073/1995) proposes a composition containing oxide (Y.sub.2 O.sub.3) in a predetermined concentration range (1.times.10.sup.-4 to 2.times.10.sup.-3 in mol number) and describes that the the brightness is 1.5 times higher than that of a phosphor having a composition beyond the above-mentioned concentration range. The latter publication (Japanese Unexamined Patent Publication No. 310074/1995) teaches a composition ((Y.sub.1-x-y La.sub.x Ln.sub.y).sub.2 O.sub.2 S): where Ln being Eu and/or Sm, 0&lt;x.ltoreq.0.035; 0.01.ltoreq.y.ltoreq.0.1) obtained by replacing a part of the element Y by La and describes that the composition achieves a brightness 1.5 times that of a composition containing no La. As apparent from the description in each of the above-mentioned publications, it is revealed, by varying the mixing ratio of a plurality of materials required to synthesize the phosphor and the synthesizing condition (specifically, the heating and baking temperature and the holding time for the synthesis within a crucible) and examining resultant phosphors for brightness and composition, that a relatively high brightness is achieved in a particular composition range. Thus, the above-mentioned publications are noticeable in that those phosphors having been neglected as being impractical can be improved with slight modification in composition to exhibit a high brightness. However, as described in conjunction with the prior art, those skilled in the art often experience that, even if the low voltage electron beam excitation phosphor display apparatus is manufactured by the use of the phosphor within the particular composition range, a desired light-emission characteristic is not obtained. If it is assumed that the desired light-emission characteristic is obtained by specifying the composition range, then a great problem is still present for practization. Specifically, it is necessary to find what manufacturing method or manufacturing condition is to be adopted in order that the composition of the phosphor finally obtained in the display apparatus via the manufacturing process falls within the above-mentioned particular composition range. In the above-mentioned two publications, no mention is made about the manufacture of the display apparatus. This leaves the problem in practization of the display apparatus. Admittedly, each publication is novel in that the composition of the phosphor contains no cadmium. However, comparison in brightness with the conventional red-based phosphor (ZnCd)S is not disclosed. It is therefore presumed that the emission brightness sufficient for replacement can not be obtained.
The prior art has following problems.
(a) In the prior art, (Zn,Cd)S containing cadmium which is a special chemical substance is widely used as the red-based phosphor. Thus, the prior art has problems such as environmental pollution. In recent years, cadmium-free phosphors have been disclosed. Such disclosure is useful in producing the phosphor. On the other hand, no disclosure is made about a practical method of manufacturing the display apparatus. Thus, the problem is left in manufacture of the display apparatus. In addition, the above-mentioned technical disclosure does not include any comparison in brightness with the conventional phosphor (Zn,Cd)S. Consequently, it is judged that the effect of the high-brightness emission sufficient for replacement is not yet achieved. Thus, the prior art does neither disclose a highly practical novel phosphor free from cadmium and capable of performing high-brightness red-based emission nor disclose the method of manufacturing the low voltage electron beam excitation phosphor display apparatus using the novel phosphor. PA1 (b) In order to perform multi-color display using a plurality of emission colors (for example, R, G, and B) with high brightness, use is made of phosphors exhibiting different emission colors, as will readily be understood. In the prior art, those phosphors performing high-brightness light emission in response to the low voltage electron beam are restricted. As a result, use has been made of a plurality of types of the phosphors which are different compounds. Therefore, in manufacture of a multi-color display apparatus, it is impossible to individually determine an optimum manufacturing process or condition corresponding to each phosphor. Instead, pilot run is experimentally carried out in the common manufacturing process and condition or in several kinds of settings close thereto. Evaluation is then carried out and fluctuation factors are considered to determine a final setting. PA1 (c) In the display apparatus for providing multi-color display by mixture of a plurality of emission colors, difference in emission characteristic resulting from fluctuation factors in manufacture, particularly, slight difference in emission brightness results in occurrence of color offset in display, as known in the art. In this event, the display apparatus is extremely poor in color reproducibility. In the prior art using the low voltage electron beam in which different phosphors are inevitably required and result in a great problem in manufacture, variation in emission brightness is different in dependence upon each type of the phosphor because of difference in chemical nature of the phosphors, even if subjected to the same fluctuation factor during manufacture. Thus, color offset is inherently easy in this structure. The above-mentioned problem in manufacture is particularly serious in the FED for displaying a full-color video image as a mixture of the three emission colors (R, G, B) in the manner similar to the television. PA1 (d) As described above, the phosphor is degraded in emission brightness by lighting for a long time and has a so-called emission lifetime (desirably, at least several thousand hours, more desirably, at least twenty or thirty thousand hours). The different types of the phosphors do not have the same lifetime. In the phosphor technique related to the fast electron beam such as for the television, the above-mentioned problem has been solved. On the other hand, in the phosphor technique related to the low voltage electron beam, no solution has been reached yet, as described above. In the prior art, the multi-color display apparatus using different phosphors for mixture of the emission colors from the respective phosphors inherently has a following problem. During a long time usage (for example, for several years), phosphors are degraded their light emission ability due to the electron bombardment of long term, and diminish the brightness. Moreover, the degradation generally differs in phosphors of a different compound. A brightness unbalance, thus caused, among R, G, B colors are easily observed, in the case of video movies, as pictures with a poor color balance.
As mentioned above, in multi-color display in the prior art, different phosphors are inevitably required. It is therefore difficult to determine the setting in manufacture so as to achieve an optimum light-emission characteristic for each phosphor. As a result, it is impossible to manufacture the display apparatus fully utilizing the light-emission characteristics (brightness, color tone, emission efficiency, lifetime) of each phosphor.